Due to the advancement of manufacturing technology, light emitting diodes (LEDs) have been widely used in solid-state lighting products. According to physical characteristics of the LED, the LED is mostly suggested to be supplied with low voltage and direct current. Thus, LED is usually incorporated with step-down transformer and AC-to-DC converter whenever only high-voltage alternating current is available. However, the transformer and the converter usually occupy a large volume and are too heavy for the user to carry around. In addition, as alternating current is transformed into direct current, it is always accompanied with the problem of energy waste. Thus, it is crucial for the manufacturers to provide LED related products with higher efficiency.